1. Field of the Invention
The present invention relates to data communications, and, more particularly, efficiency in data communication circuits.
2. Description of the Related Art
A data communications network is the interconnection of two or more communicating entities (i.e., data sources and/or sinks) over one or more data links.
A data communications network allows communication between multiple communicating entities over one or more data communications links. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, these demands are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation, and capacity for future growth.
The synchronous optical network (SONET) protocol is one among those protocols designed to employ an optical infrastructure and is widely employed in voice and data communications networks. SONET is a physical transmission vehicle capable of transmission speeds in the multi-gigabit range, and is defined by a set of electrical as well as optical standards. A similar standard to SONET is the Synchronous Digital Hierarchy (SDH) which is the optical fiber standard predominantly used in Europe. There are only minor differences between the two standards. Accordingly, hereinafter any reference to the term SONET refers to both SDH and SONET networks, unless otherwise noted.
In some networks, network nodes store data which they use for proper operation. In SONET, data between adjacent nodes are transmitted in modules called STS""s (synchronous transport signals). Each STS is transmitted on a link at regular time intervals (for example, 125 microseconds). See Bellcore Generic Requirements document GR-253-CORE (Issue 2, December 1995), hereinafter referred to as xe2x80x9cSONET Specification,xe2x80x9d and incorporated herein by reference for all purposes.
SONET network equipment transmits the STS frames in various paths termed Line, Section, and Path. Referring to FIG. 1A, SONET equipment is shown as depicting Section, Line and Path definitions. Path Terminating Equipment 10 is shown coupled to Line Terminating Equipment 20. Line Terminating Equipment 20 is coupled to Section Terminating Equipment 30. In general, the network equipment shown in FIG. 1A includes fiber optic equipment that interfaces with other types of transmission equipment.
When transported, the signals are broken into layers: Physical, Section, Line and Path. The layers are hierarchical in nature, with each layer performing a different function. The Physical layer addresses the transport of bits across the physical medium. Accordingly, no overhead is associated with the physical layer. The equipment associated with the Physical layer converts STS-N signals into optical or electrical SONET signals. The Section layer addresses the transport of STS-N frames across the physical medium. The equipment associated with this layer functions to perform framing, scrambling, error monitoring and section-level overhead. Section Terminating equipment 30 interprets, modifies and creates section overhead. The Line layer addresses transport of Path level payloads. The Line layer further synchronizes and multiplexes functions for the Path layer. Line layer overhead functions to maintain and protect. The overhead is interpreted, modified and created by Line Terminating Equipment 20. The Path layer addresses transporting payloads between SONET terminal multiplexing equipment. The Path layer maps payloads into formats required by the Line layer. The Path layer communicates xe2x80x98end-to-end with the Path overhead with Path Terminating Equipment 10. Generally, network equipment that contains Path Terminating equipment also contains Section and Line Terminating equipment.
Referring to the Section Overhead, the SONET Specification provides that the section overhead bytes A1 and A2 are allocated in each STS-1 for framing. Specifically, the Specification provides that the A1 byte is set to xe2x80x9811110110xe2x80x99 and the A2 byte is set to xe2x80x9800101000xe2x80x99 in all STS-1s in an STS-N. Framing is accomplished by network equipment finding the transition between the A1 and A2 bytes. Thus, finding the transition provides the second byte position, and finding the second byte position provides enough data to frame an entire SONET STS-1 frame, thereby aligning the data as it passes downstream.
A problem with the current SONET/SDH rules for framing is that newer SONET STS frames have grown larger with newer technology. Following the SONET Specification, framing requires receiving 192 bytes of an A1 byte and 192 bytes of an A2 byte for a OC-192 frame. There are several problems associated with the method provided in the Specification. Among those problems, there is a probability that the bytes found by a framer are not A1 or A2 bytes, because the probability of false framing increases in proportion to the number of bits considered. Further, the increased time necessary for determining frames using the method defined in the Specification is inefficient. What is needed is an efficient system and method for framing high-speed signals.
Accordingly, a method for framing an incoming bit stream provides high-speed signal framing. The method includes receiving the incoming bit stream in a datapath and locating a predetermined framing pattern in the datapath. More specifically, the locating includes finding a predetermined number of repetitions of a first portion of the framing pattern, bit aligning the bits in the datapath based on the predetermined number of repetitions of the first portion, priority encoding bits in a next cycle of the datapath, identifying a location of a second portion of the framing pattern, word aligning the priority encoded bits. The method continues with declaring the bit stream as in frame.
According to an embodiment, the incoming bit stream is over a datapath of at least 64 bits and the predetermined number of repetitions is at least three repetitions. Further, in the embodiment, the incoming bitstream is a parallelized bitstream, the parallelization being performed in a shift register.
In one embodiment, the incoming bitstream is a synchronous transport signal (STS) having N modules (STS-N). N is variable and in one embodiment N is 192.
Another embodiment is directed to an apparatus disposed in a communication system. The apparatus includes a shift register to receive an incoming bitstream that is configured to parallelize the incoming bitstream. The apparatus further includes a bit-aligning multiplexer coupled to the shift register, the bit-aligning multiplexer bit-aligning the parallelized bitstream according to a first portion of a framing pattern. The apparatus further includes a priority encoder coupled to the bit-aligning multiplexer, the priority encoder locating a priority byte in the parallelized bitstream identified in a second portion of the framing pattern. The priority encoder further identifies a transition between the first portion and the second portion of the framing pattern. The apparatus further includes a byte-aligning multiplexer coupled to the priority encoder, the byte-aligning multiplexer byte-aligning the parallelized bitstream according to the priority byte, the byte-aligning determining frame borders of the incoming bitstream.
According to an embodiment, the bit-aligning multiplexer bit aligns three repetitions of the first portion of the framing pattern. Further, the priority encoder includes eight comparators coupled to a multiplexer. In one embodiment, the apparatus is disposed on an application specific integrated circuit (ASIC) coupled to a line card, the line card being one of a plurality of line cards disposed in a management bay holding one or more line cards configured to transmit a plurality of signals
Another embodiment of the invention is directed to a computer program product for communication. The computer program product includes signal bearing media bearing programming adapted to locate a predetermined framing pattern with the bits in an incoming datapath, find a predetermined number of repetitions of a first portion of the framing pattern, bit align the bits in the datapath based on the predetermined number of repetitions of the first portion, priority encode bits in a next cycle of the datapath, identify a location of a second portion of the framing pattern, word align the priority encoded bits, and declare the aligned datapath as in frame. The predetermined number of repetitions is three in one embodiment.
Another embodiment is directed to a communication system including means for locating a predetermined framing pattern with the bits in an incoming datapath, the means for locating including: means for finding a predetermined number of repetitions of a first portion of the framing pattern, means for bit aligning the bits in the datapath based on the predetermined number of repetitions of the first portion, means for priority encoding bits in a next cycle of the datapath, means for identifying a location of the second portion of the framing pattern, and means for word aligning the priority encoded bits. The method further includes means for declaring the bit stream as in frame.